Jet ink printing systems discharge discrete droplets of ink through a fine jet nozzle to the substrate. In this method of printing, unlike letter press, gravure, silk screen and comparable printing techniques, there is no contact between the printer and article to be printed. The ink droplets have a defined resistivity so that they can be deflected by an electric field when discharged from the nozzle. The jet ink printing system is of considerable importance in applying decorative and identifying indicia to a variety of substrates, including glass, metal and synthetic polymeric surfaces. U.S. Pat. Nos. 3,465,350 and 3,465,351 are exemplary of these systems.
Ink to be used with jet printing means have specific viscosity and resistivity limitations, should provide good wettability of the substrate, and must be quick drying and smear resistant. Further, the ink must be compatible both with the jet printing equipment and in the eventual end use application. With regard to the former it is critical that the ink flow through the fine jet nozzles without clogging same.
Typically, jet inks consist of three basic components--a colorant such as a dye or pigment; a resin binding agent which serves to secure or adhere the colorant to the substrate surface, and a carrier fluid or solvent for the colorant or binding agent, the carrier fluid evaporating upon application of the ink. In jet printing inks, an electrolyte is often employed to ensure that the droplets can be adequately charged whereby proper deflection is achieved. U.S. Pat. No. 4,021,252 issued to Banczak et al is representative of this genre of jet printing inks. Banczak uses from one to 25% by weight shellac as the binding agent, between 0.5 and 5% by weight of a basic dye as the colorant, and a solvent blend consisting of water and alcohol. Optionally the solvent blend may be modified by the addition of a glycol ether to solubilize colorants of limited solubility.
Other prior art patents reflective of the Banczak approach are listed below, some of which are for specific end use applications:
______________________________________ U.S. Pat. No. 4,024,096 Wachtel 4,070,322 Hwang et al 4,155,767 Specht et al 4,155,895 Rohowetz et al 4,168,254 Fell 4,168,662 Fell 4,177,075 Mansukhani 4,186,020 Wachtel 4,196,006 Mansukhani 4,196,007 Mansukhani German Offenlegungsschrift 28 12 364 M&T Chemicals (Published October 1978) ______________________________________
To produce an opaque white image utilizing the ink formulations of the above identified prior art references, it would be necessary to use a pigment such as rutile titanium dioxide. However, this opacifying agent is present in the solvent as a dispersed solid, and tends to settle out of solution thereby clogging the jet nozzle. This drawback is even more pronounced in low viscosity jet inks which are preferred. Other disadvantages of conventional pigmented inks are that they usually require curing at elevated temperatures, and/or have extended drying times. The goal of the present invention, then, is to provide non-pigmented jet inks that produce opaque white images on non-porous substrates, and which can be applied to said substrates in a single operating step.
As is well known in the art, a solution of certain compounds, particularly the cellulose esters, when applied to a surface will dry opaque white. See generally, Jerome Seiner, Microvoids As Pigments. A Review, Industrial and Engineering Chemistry, Product Research & Development, Vol. 17, pp. 302-317 (Dec. 12, 1978) and J. J. Clancy, Microvoid Coatings in Graphic Art Application. A Patent Survey, Industrial and Engineering Chemistry, Product Research & Development, Vol. 13, No. 1, pp. 30-34 (March 1974). A sampling of patents issued with respect to such compositions follows:
______________________________________ U.S. Pat. No. 1,449,157 Wilkie 2,296,337 Cummings 2,519,660 James 2,739,909 Rosenthal 2,927,039 Vander Weel 3,020,172 Mohnhaupt 3,031,328 Larsen 3,654,193 Seiner 3,655,591 Seiner ______________________________________
The above inventions are related to paints, paper coatings, and recording materials.
More recently, U.S. Pat. No. 4,207,577 to Mansukhani applied this technology to jet inks. The Mansukhani patent discloses a two-step procedure for obtaining opacity, the second step being either an adjustment of the temperature of the applied film to the ambient dew point, or by applying moisture thereto. In an alternate embodiment a basic dye is used in the ink formulation, the film then being moistened with water. The method above is applied to a general ink composition consisting of cellulose esters, binding agents and a solvent blend. The two step approach of Mansukhani represents a serious limitation in the use of his ink in certain applications. For example, some products are sensitive and can not undergo the second "moisturing" step. Other products are humidity sensitive. Similarly, printing on a hot surface will drive off the solvents before the second step can be accomplished. Finally, the Mansukhani method requires the addition of a second step treatment zone to existing product conveying systems.